The present invention relates to earth-penetrating drill bits, and particularly to sealing structures in so-called roller-cone bits.
Background: Rotary Drilling
Oil wells and gas wells are drilled by a process of rotary drilling, using a drill rig such as is shown in FIG. 3. In conventional vertical drilling, a drill bit 110 is mounted on the end of a drill string 112 (drill pipe plus drill collars), which may be several miles long, while at the surface a rotary drive (not shown) turns the drill string, including the bit at the bottom of the hole.
Two main types of drill bits are in use, one being the roller cone bit, an example of which is seen in FIG. 2. In this bit a set of cones 116 (two are visible) having teeth or cutting inserts 118 are arranged on rugged bearings. As the drill bit rotates, the roller cones roll on the bottom of the hole. The weight-on-bit forces the downward pointing teeth of the rotating cones into the formation being drilled, applying a compressive stress which exceeds the yield stress of the formation, and thus inducing fractures. The resulting fragments are flushed away from the cutting face by a high flow of drilling fluid.
The drill string typically rotates at 150 rpm or so, and sometimes as high as 1000 rpm if a downhole motor is used, while the roller cones themselves typically rotate at a slightly higher rate. At this speed the roller cone bearings must each carry a very bumpy load which averages a few tens of thousands of pounds, with the instantaneous peak forces on the bearings several times larger than the average forces. This is a demanding task.
Background: Bearing Seals
In most applications where bearings are used, some type of seal, such as an elastomeric seal, is interposed between the bearings and the outside environment to keep lubricant around the bearings and to keep contamination out. In a rotary seal, where one surface rotates around another, some special considerations are important in the design of both the seal itself and the gland into which it is seated.
The special demands of sealing the bearings of roller cone bits are particularly difficult. The drill bit is operating in an environment where the turbulent flow of drilling fluid, which is loaded with particulates of crushed rock; is being driven by hundreds of pump horsepower. The flow of mud from the drill string may also carry entrained abrasive fines. The mechanical structure around the seal is normally designed to limit direct impingement of high-velocity fluid flows on the seal itself, but some abrasive particulates will inevitably migrate into the seal location. Moreover, the fluctuating pressures near the bottomhole surface mean that the seal in use will see forces from pressure variations which tend to move it back and forth along the sealing surfaces. Such longitudinal xe2x80x9cworkingxe2x80x9d of the seal can be disastrous in this context, since abrasive particles can thereby migrate into close contact with the seal, where they will rapidly destroy it.
Commonly-owned U.S. application Ser. No. 09/259,851, filed Mar. 1, 1999 and now issued as U.S. Pat. No. 6,279,671 (Roller Cone Bit With Improved Seal Gland Design, Panigrahi et al.), copending (through continuing application Ser. No. 09/942,270 filed Aug. 27, 2001 and hereby incorporated by reference) with the present application, described a rock bit sealing system in which the gland cross-section includes chamfers which increase the pressure on the seal whenever it moves in response to pressure differentials. This helps to keep the seal from losing its xe2x80x9cgripxe2x80x9d on the static surface, i.e. from beginning circumferential motion with respect to the static surface. FIG. 4 shows a sectional view of a cone according to this application; cone 116 is mounted, through rotary bearings 12, to a spindle 117 which extends from the arm 46 seen in FIG. 1. A seal 20, housed in a gland 22 which is milled out of the cone, glides along the smooth surface of spindle 117 to exclude the ambient mud 21 from the bearings 12. (Also visible in this Figure is the borehole; as the cones 116 rotate under load, they erode the rock at the cutting face 25, to thereby extend the generally-cylindrical walls 25 of the borehole being drilled.) The present application discloses a different sealing structure, in place of the seal 20 and gland 22, but FIG. 4 gives a view of the different conventional structures which the seal protects and works with.
Optimized Earth Boring Seal Means
The present application teaches a seal gland having a contour which is designed to achieve a particular stress distribution in relation to the DEFORMED seal, in its installed position. In the presently preferred embodiment, the stress distribution includes not only sealing stress areas (on both the journal and the gland sides), but also an area of distributed preload stress in substantially all of the moving area (on the xe2x80x9cdynamicxe2x80x9d side of the seal) which laterally retains the seal. The areas of distributed preload stress provide a mild preloading for the installed seal, so that longitudinal forces (due to differential pressure) merely produce an increased stress in these areas, without inducing motion. The peak value of this preload stress is preferably minimized, to avoid friction and/or seal erosion, and the minimum value of this stress is preferably kept above zero, to avoid in-migration of particulates.
Simulation of the seal""s deformed profile is preferably used to estimate the distribution of stresses. The locations and dimensions of the sealing surfaces, of the gland, and of the seal will define an initial value for sealing stress, as well as an initial value for preload stress if any. The contour (and possibly dimensions) of the retainer lip can then be adjusted as appropriate, to achieve the distribution of preload stress described above.
The contour of the seal under load will depend on the seal""s unloaded cross-section, and on the load which is applied to it by the contour of the metal elements it is interfaced to. Thus achievement of a uniform preload stress in the longitudinal retention areas actually requires solution of a variational problem, since the contour of the metal shapes and the as-deformed seal contour are both variables which must be jointly optimized to achieve the desired result.